Building Blocks: Retaining Walls
I had never really noticed the hill before. One day, while attending a barbecue party at my in-laws’, I happened to look up, and was struck by the giant mound of earth looming above their backyard, extending about seventy-five feet above the patio.
In front of that mound was a concrete block wall. I remember thinking, “I hope that wall never fails, because the hill would take the house out with it.”
That was a quite a few years ago. One earthquake and several pounding rainstorms later, that wall—and the hill, and the house—are all still in place. That’s a testament to the skill and knowledge of the landscape contractor who built the strong, six-foot-high masonry retaining structure that’s holding that hill back.
The definition of the word ‘retain’ is ‘to continue to keep possession of something.’ In this case, that ‘something’ is soil. A retaining wall is supposed to hang on to soil, so that it doesn’t inundate whatever’s next to or under it, like someone’s backyard or house.
If you’re a landscape contractor who does design/build, you’ll probably be called upon to build quite a few retaining walls in your career. It’s important to know how to build them right.
While these structures have specific, very important jobs, they don’t have to look drab and utilitarian; they can also be beautiful. Retaining walls are often found in outdoor living areas, where they not only hold back soil, but multitask as built-in seating areas, planters, or as backdrops for water features or fire pits.
In fact, the burgeoning demand for these spaces is driving the market for retaining walls and the products used to build them.
Retaining walls have been, and still are, built out of all kinds of things: timbers—including reclaimed railroad ties, boulders, natural stone, clay brick, cast-in-place concrete, precast concrete block, gabions (rock-filled wire baskets) and segmental concrete block.
There’s even a product made of expanded polystyrene called ‘geofoam,’ that’s extremely light but very strong, used in certain commercial applications where added weight would be an issue.
Timber is one of the cheaper options for retaining walls, and people still request it despite its limited lifespan. A wooden wall will inevitably succumb to rot, but a well-built one should last 20 years or so.
Repurposed wooden railroad ties were a popular retaining wall material a few decades ago, and are still sold in a few places. But, according to the EPA, they should be avoided. They’re heavily treated with creosote and arsenic, and as they decay, release those toxins into the environment. Anyone handling or sawing into them without taking special precautions risks exposure to these chemicals, some of which are carcinogenic.
“We refuse to build anything out of rail ties anymore,” said Aaron Wiltshire, owner and president of Oklahoma Landscape, Inc., in Tulsa. “If a customer wants that, we try to steer them towards a different product.”
Clay brick is very labor-intensive, placing it among the more expensive options, along with quarried, cut natural stone. When clay brick is used, it’s generally for shorter retaining walls (sometimes called ‘gravity’ walls), generally no taller than three feet.
Walls composed of boulders are very attractive and among the less expensive options. The voids between them allow for drainage.
Wiltshire has been building boulder walls for more than 35 years.
“They make great wall systems if a client wants something more organic, less structured-looking. You can build stairs, and terraces, and all kinds of neat things out of boulders. In Oklahoma, they cost about the same as the segmental systems.”
Stan ‘Dirt Monkey’ Genadek owns Genadek Landscaping and Excavating, Inc., in St. Paul, Minnesota, and trains and coaches contractors through his Dirt Monkey University. He builds a lot of retaining walls out of boulders, as well as segmental concrete block systems and concrete masonry units (CMU).
He says the engineering for a boulder wall is similar to that of a segmental or masonry wall. “We also put a heavy woven geofabric behind the boulders, unless the homeowner agrees to plant the joint faces, even before the job is done.”
Essentially, that creates a boulder backed green wall. “Plant roots make a phenomenal anchoring material. Roots mat the soil together better than boulder fabric can, and last longer—but it’s a lot more work if you go that route.”
You do have to be careful, though, about what plants you choose to vegetate a wall with, says Genadek. For instance, you don’t want to plant a tree on top of any retaining wall. As the tree grows, its weight adds to the wall’s surcharge. Also, as its roots extend down, they will push on the wall, and eventually, cause it to fail.
Low-growing bushes and shrubs, and ground-cover plants that spread and grow in between surfaces should be chosen instead. They’ll literally knit the wall together.
Segmental concrete block
Today, a great many retaining walls, in both residential and commercial settings, are being constructed from segmental concrete block, either solid or hollow-core. It’s made by many different manufacturers, in a variety of colors and textures, with faces that can be molded to mimic the appearance of natural stone.
They’re manufactured to ‘drystack,’ and are interlocked via either a lip or pin system, depending on the manufacturer. A lip is a formed channel in the block itself that locks the blocks together. In a pin system, the blocks come with holes or slots where the pins, made of proprietary metal or fiberglass-reinforced nylon, are inserted.
Their popularity has soared ever since they started appearing on the market about twenty years ago. They’re much cheaper than almost anything else you can build a wall out of nowadays, despite a higher initial cost than some other materials. What makes them economical is the speed of construction. Not having to apply mortar, insert rebar, pour concrete into molds—or wait for it to dry—saves on labor costs.
“We build a lot of retaining walls out of segmental systems, because they’re so affordable,” said Wiltshire. “A large niche of our current client base likes them. They’re decorative, and long-lasting, if they’re installed correctly. About 60 to 70 percent of our retaining walls are segmental.”
“The driver for the segmental products at the residential level is really their aesthetics, how they can coordinate someone’s entire outdoor living space and the rest of the landscape,” said Joe Raboine, a design and training specialist for Belgard, Atlanta, Georgia.
“In almost all parts of the country, the install price for segmental retaining walls is far less expensive than masonry with a structural footing, or cast-in-place concrete,” said Karen Nelson, P.E., engineering services manager at Versa-Lok in Oakdale, Minnesota. “The taller the wall, the more competitive segmental products become.”
Segmental concrete block systems are available in both solid and hollow-core. The hollow blocks are filled with aggregate onsite to add weight and stability. Because they’re hollow, they’re cheaper to ship, and once in place, can’t be distinguished from the solid type.
Wiltshire has used both solid and hollow-core segmental blocks with success. Genadek, however, only uses the solid variety. “I don’t like a hollow block, because you’re inviting water to pass through the center. It’s a corrosive, weathering agent that will undermine the blocks from inside.”
The textured face is created by making two units at the same time, and splitting them. Blocks can even be split onsite a second time, with a special guillotine-like hydraulic or mechanical splitter device. This allows a contractor to create different types of angles and follow curves.
Genadek says that one of the good things about segmental walls is their flexibility. Because of the way they’re constructed, they’re able to ‘roll with the punches,’ and move with the earth as it shifts and settles.
“A masonry wall with mortared joints, on the other hand, will start to show cracks in the masonry, and in the mortar, as the ground settles.”
Built to last?
Contractors are often called upon to clean up after someone else’s mistakes. Wiltshire recalls one memorable example. “It was a seven- or eight-foot tall segmental wall, and whoever built it didn’t put enough of the wall below ground to hold it in correctly. Then, they got a good five-inch rain event, and voilà, there it went.”
He’s seen failed walls that were built with no provision for drainage, or on poorly compacted subbases. “I’ve seen situations where all the rocks from someone’s boulder wall are in the swimming pool. The customer is shocked; and a lot of times, the guy who built the thing won’t answer his phone.”
“There are three factors that a retaining wall design should take into consideration,” said Genadek. “The first is the soil; the second is the surcharge (the weight of the soil that the wall is holding back) at the top; and the third is the slope above and below the wall.”
A two- or three-foot retaining wall can be built without too much fuss. Things get more complicated, though, the taller a wall gets. Some states, Massachusetts being one of them, mandates that any retaining wall over four feet in height must be designed by an engineer.
“In residential settings, engineers generally aren’t consulted about retaining walls,” said Raboine. “But we encourage our contractors, anytime they’re building one that’s over three or four feet, to talk to an engineer. It adds cost, but it’s worth it, if only for their peace of mind.”
Soil, trenching and aggregate
A retaining wall, like a plant, needs to be rooted in good soil in order to succeed. For shorter walls in residential settings, the condition of the soil isn’t as critical as it is in commercial applications.
“On bigger projects, it’s standard to have a geotechnical soils report done,” said Nelson. “Knowing what kind of soil you’re dealing with is very important, because if you don’t, you’ll have to ‘overdesign,’ meaning that you must assume the worst-case scenario.”
The taller you get, the more wall you’ll need to put below ‘finish grade,’ the height of the grade in front of the wall, said Wiltshire. “You’ve got to dig a nice trench, compact the subgrade, put in aggregate, compact that, and then, set your first course (layer) well below grade.”
You might end up with two or three courses below grade, to really lock the wall into the ground. In colder areas, the footing should extend below the frost line.
Base trenches are usually two feet wide and extend the entire length of a wall. The depth will vary, but generally it’s six inches, plus an additional inch for every foot of wall height. After the trench is dug out, the soil at the bottom should be compacted until firm. Heavy clay or wet soil may need to be removed and replaced with granular aggregate.
Stan Hoglund, owner of Hoglund Landscape and Biobarrier, Inc., in Fargo, North Dakota, goes an extra step. “On any type of concrete block, stone or boulder wall, after we excavate and compact, we add a layer of stabilization cloth (geofabric), like they do on highways.”
“We put it all the way through the bottom, on top of the compacted material, and also up the sides of the trench. It keeps the compacted material from integrating into the subsoils on either side. That prevents the little dips you often see on the tops of retaining walls after a while.”
Genadek says that pea gravel should never be used as a base material. “Yes, it’s a great drainage material, but that’s where its benefits end. Those small round stones don’t have the ability to lock in place as water is passing around them. They tend to roll and shift, and as they do, they loosen the soil, causing the wall to settle, creating voids and gaps.”
Behind the wall
Just as, or even more important than what’s under a retaining wall, is what’s behind it. All that heavy earth must be stabilized, or it will eventually push the wall down.
That’s where geogrid comes in. The geosynthetic polymer reinforcement material is produced in various strengths, comes in rolls, and looks like coated chicken wire or snow fencing. It’s put in between wall courses at regular intervals, and extends back into the soil, ingeniously using the weight of the soil itself to help hold the wall up. The wall’s height determines how long the lengths of geogrid should be.
“We very seldom use geogrid in a wall that’s less than three or four feet,” said Wiltshire. “But, there can be some four-foot walls where we feel it’s necessary to put in at least one course of it. Once we get above four feet, we’re definitely using it.”
Versa-Lok specifies that geogrid should go into a wall every fourth course, or every two feet of height, because its units are six inches high. “After you set the geogrid back into the earth, you run the compactor on top of it again,” adds Wiltshire.
Water is a retaining wall’s worst enemy, and the number-one cause of failure. “You don’t want any water to be trapped behind the wall,” said Genadek. “If water gets funneled behind it, it’s practically the same as sticking a garden hose back there. It’ll blow the wall right out.”
That’s why the importance of good drainage can’t be overemphasized. If the construction method and material you’re using doesn’t provide natural voids, then water has got to have some other means of escape. A good solution is to install a French drain along the base—essentially, drain tile (perforated PVC pipe), set in a trench and covered with gravel.
Building retaining walls can help put your business on a solid footing. Build them right, and they should stand the test of time.